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Procedures at NREL for Evaluating Multijunction Concentrator Cells

Description: The procedures for evaluating the performance of multijunctiion-concentrator cells at the National Renewable Energy Laboratory are described. The accurate measurement of the performance of multijunction cells requires accurate relative-quantum-efficiency-measurements, "matched" reference cells, and a spectrally adjustable solar simulator.
Date: January 1, 2000
Creator: Moriarty, T. & Emery, K.
Partner: UNT Libraries Government Documents Department

Thermophotovoltaic Cell Temperature Measurement Issues

Description: The power produced by photovoltaic devices changes with temperature, ranging from 0.1% to nearly 1% per degrees Celsius depending on the structure. The temperature across the surface of TPV cells will vary depending on the amount of absorbed power. Thus the temperature over a region of a wafer where there is no cell will be different from a region of the wafer containing a cell with an antireflection coating and back surface reflector. Vacuum hold-downs or back surface probes may result in local hot spots. Bonding a cell to a heat sink may not be practical in a research environment, and a temperature gradient between the heat sink and space-charge region will still exist. Procedures for determining the current versus voltage (I-V) characteristics at a given temperature are discussed. For continuous illumination measurement systems, the temperature of the heat sink or backside of the device can be directly measured. The temperature can also be inferred by placing the sample at a known temperature in the dark, and monitoring the open-circuit voltage (Voc) as a high-speed shutter is opened. The maximum Voc from this method corresponds to the temperature in the dark and the plate temperature can then be lowered until this maximum Voc is reached. The temperature can also be indirectly determined from the dark I-V characteristics, assuming negligible series resistance in the ideal case that the voltage in the dark at a given current and temperature corresponds to the Voc and short circuit voltage (Isc) at that temperature. A high-intensity flash simulator will produce negligible cell heating during the flash and therefore the cell temperature may be easily set before the flash.
Date: November 13, 1998
Creator: Emery, K. & Moriarty, T.
Partner: UNT Libraries Government Documents Department

Algorithm for Building a Spectrum for NREL's One-Sun Multi-Source Simulator: Preprint

Description: Historically, the tools used at NREL to compensate for the difference between a reference spectrum and a simulator spectrum have been well-matched reference cells and the application of a calculated spectral mismatch correction factor, M. This paper describes the algorithm for adjusting the spectrum of a 9-channel fiber-optic-based solar simulator with a uniform beam size of 9 cm square at 1-sun. The combination of this algorithm and the One-Sun Multi-Source Simulator (OSMSS) hardware reduces NREL's current vs. voltage measurement time for a typical three-junction device from man-days to man-minutes. These time savings may be significantly greater for devices with more junctions.
Date: June 1, 2012
Creator: Moriarty, T.; Emery, K. & Jablonski, J.
Partner: UNT Libraries Government Documents Department

Formation of ZnTe:Cu/Ti Contacts at High Temperature for CdS/CdTe Devices (Presentation)

Description: The conclusions of this report are that Cu diffusion from a ZnTe:Cu contact causes good and bad things. The good (Cu in CdS < low 10{sup 18} cm{sup -3})--increase in CdTe N{sub A}-N{sub D} that leads to V{sub oc} and FF improvement. The bad (Cu in CdS > low 10{sup 18} cm{sup -3})--(1) possibly decreased of shunt resistance (?); (2) depletion width in CdTe can become too narrow for optimum current collection at J{sub MPP}; (3) donor reduction in CdS (significant FF loss in LIV); and (4) excessive Cu diffusion into CdS readily observed by red-light bias QE.
Date: May 1, 2006
Creator: Gessert, T. A.; Asher, S.; Johnston, S.; Duda, A.; Young, M. R. & Moriarty, T.
Partner: UNT Libraries Government Documents Department

PV Cell and Module Calibration Activities at NREL

Description: The performance of PV cells and modules with respect to standard reference conditions is a key indicator of progress of a given technology. This task provides the U.S. terrestrial PV community with the most accurate measurements that are technically possible in a timely fashion. The international module certification and accreditation program PVGap requires certification laboratories to maintain their calibration traceability path to groups like this one. The politics of a "world record" efficiency requires that an independent laboratory perform these measurements for credibility. Most manufacturers base their module peak watt rating upon standards and reference cells calibrated under this task. This task has been involved in reconciling disputes between manufacturers and their cell suppliers in terms of expected versus actual performance. This task has also served as a resource to the PV community for consultation on solar simulation, current versus voltage measurement instrumentation, measurement procedures and measurement artifacts.
Date: November 1, 2005
Creator: Emery, K.; Anderberg, A.; Kiehl, J.; Mack, C.; Moriarty, T.; Ottoson, L. et al.
Partner: UNT Libraries Government Documents Department

Formation of ZnTe:Cu/Ti Contacts at High Temperature for CdS/CdTe Devices: Preprint

Description: We study the performance of CdS/CdTe thin-film devices contacted with ZnTe:Cu/Ti of various thickness at a higher-than-optimum temperature of {approx}360 C. At this temperature, optimum device performance requires the same thickness of ZnTe:Cu as for similar contacts formed at a lower temperature of 320 C. C-V analysis indicates that a ZnTe:Cu layer thickness of {approx}< 0.5 mu m does not yield the degree of CdTe net acceptor concentration necessary to reduce space charge width to its optimum value for n-p device operation. The thickest ZnTe:Cu layer investigated (1 mu m) yields the highest CdTe net acceptor concentration, lowest value of Jo, and highest Voc. However, performance is limited for this device by poor fill factor. We suggest poor fill factor is due to Cu-related acceptors compensating donors in CdS.
Date: May 1, 2006
Creator: Gessert, T. A.; Asher, S.; Johnston, S.; Duda, A.; Young, M. R. & Moriarty, T.
Partner: UNT Libraries Government Documents Department

PV cell and module performance measurement capabilities at NREL

Description: The Photovoltaic (PV) Cell and Module Performance Characterization team at NREL supports the entire photovoltaic community by providing: secondary calibrations of photovoltaic cells and modules; efficiency measurements with respect to a given set of standard reporting conditions; verification of contract efficiency milestones; and current versus voltage (I-V) measurements under various conditions of temperature, spectral irradiance, and total irradiance. Support is also provided to in-house programs in device fabrication, module stability, module reliability, PV systems evaluations, and alternative rating methods by performing baseline testing, specialized measurements and other assistance when required. The I-V and spectral responsivity equipment used to accomplish these tasks are described in this paper.
Date: September 1, 1998
Creator: Rummel, S.; Emery, K.; Field, H.; Moriarty, T.; Anderberg, A.; Dunlavy, D. et al.
Partner: UNT Libraries Government Documents Department

New GaInP/GaAs/GaInAs, Triple-Bandgap, Tandem Solar Cell for High-Efficiency Terrestrial Concentrator Systems

Description: GaInP/GaAs/GaInAs three-junction cells are grown in an inverted configuration on GaAs, allowing high quality growth of the lattice matched GaInP and GaAs layers before a grade is used for the 1-eV GaInAs layer. Using this approach an efficiency of 37.9% was demonstrated.
Date: November 1, 2005
Creator: Kurtz, S.; Wanlass, M.; Kramer, C.; Young, M.; Geisz, J.; Ward, S. et al.
Partner: UNT Libraries Government Documents Department

High-Performance, 0.6-eV, GA0.32In0.68As/In0.32P0.68 Thermophotovoltaic Converters and Monolithically Interconnected Modules

Description: Recent progress in the development of high-performance, 0.6-eV Ga0.32In0.68As/InAs0.32P0.68 thermophotovoltaic (TPV) converters and monolithically interconnected modules (MIMs) is described. The converter structure design is based on using a lattice-matched InAs0.32P0.68/Ga0.32In0.68As/InAs0.32P0.68 double-heterostructure (DH) device, which is grown lattice-mismatched on an InP substrate, with an intervening compositionally step-graded region of InAsyP1-y. The Ga0.32In0.68As alloy has a room-temperature band gap of {approx}0.6 eV and contains a p/n junction. The InAs0.32P0.68 layers have a room-temperature band gap of {approx}0.96 eV and serve as passivation/confinement layers for the Ga0.32In0.68As p/n junction. InAsyP1-y step grades have yielded DH converters with superior electronic quality and performance characteristics. Details of the microstructure of the converters are presented. Converters prepared for this work were grown by atmospheric-pressure metalorganic vapor-phase epitaxy (APMO VPE) and were processed using a combination of photolithography, wet-chemical etching, and conventional metal and insulator deposition techniques. Excellent performance characteristics have been demonstrated for the 0.6-eV TPV converters. Additionally, the implementation of MIM technology in these converters has been highly successful.
Date: December 15, 1998
Creator: Wanlass, M. W.; Carapella, J. J.; Duda, A.; Emery, K.; Gedvilas, L.; Moriarty, T. et al.
Partner: UNT Libraries Government Documents Department

Inverted GaInP/(In)GaAs/InGaAs Triple-Junction Solar Cells with Low-Stress Metamorphic Bottom Junctions: Preprint

Description: We demonstrate high efficiency performance in two ultra-thin, Ge-free III-V semiconductor triple-junction solar cell device designs grown in an inverted configuration. Low-stress metamorphic junctions were engineered to achieve excellent photovoltaic performance with less than 3 x 106 cm-2 threading dislocations. The first design with band gaps of 1.83/1.40/1.00 eV, containing a single metamorphic junction, achieved 33.8% and 39.2% efficiencies under the standard one-sun global spectrum and concentrated direct spectrum at 131 suns, respectively. The second design with band gaps of 1.83/1.34/0.89 eV, containing two metamorphic junctions achieved 33.2% and 40.1% efficiencies under the standard one-sun global spectrum and concentrated direct spectrum at 143 suns, respectively.
Date: May 1, 2008
Creator: Geisz, J. F.; Kurtz, S. R.; Wanlass, M. W.; Ward, J. S.; Duda, A.; Friedman, D. J. et al.
Partner: UNT Libraries Government Documents Department

High-Efficiency CTO/ZTO/CdS/CdTe Polycrystalline Thin-Film Solar Cells: Preprint

Description: Presented at the 2001 NCPV Program Review Meeting: The device performance and reproducibility of CdTe cells can be improved by using a modified CTO/ZTO/CdS/CdTe device structure developed at NREL.
Date: October 1, 2001
Creator: Wu, X.; Dhere, R. G.; Albin, D. S.; Gessert, T. A.; DeHart, C.; Keane, J. C. et al.
Partner: UNT Libraries Government Documents Department